The present invention relates to a sliding contact material that is used in a sliding part electrically switching on and off by a mechanical sliding action, particularly to a sliding contact material that is used in a commutator for a small direct-current motor which is used for loading of taking a CD in and out in a CD player or used for sending a pick to move a lens for reading signals of a CD, and further used in a commutator for a small direct-current motor which is used in household electrical appliances that are driven with a rechargeable battery (and others, including an earth ring and a rotary switch).
Recently, in the technical field mentioned above, studies have been energetically carried out to develop new sliding contact materials. Concerning these sliding contact materials, it may be stated that the most important subject to be developed is to make abrasion ideal when the contact is used and to realize low contact resistance. Originally, realizing the low contact resistance of a sliding contact material can be attained by surely contacting or adhering both materials to be contacted each other, as well as their conductivities. However, when the material slides, the higher the degree of the contact or adhesion of both materials to be contacted is, the larger the frictional resistance becomes, and sliding the materials against the friction produces remarkable abrasion phenomena. That is, in a sliding contact material, it is difficult to obtain more ideal properties unless the above-mentioned phenomena that are contrary to each other are controlled. Further, there are many scientifically unsolved points in the abrasion phenomena of this sliding contact, and it is also stated that controlling the abrasion phenomena by improving a sliding contact material is very difficult.
The abrasion in sliding contact materials is divided broadly into cohesion abrasion and scratch abrasion. Generally, even if the surface of a sliding contact material is finished to be considerably smooth, it is not a complete plane surface from the microstructural point of view and there are many micro uneven parts. When such metal surfaces are made contacted each other, though it seems that they are apparently contacted over the wide range of areas, they are practically in the state that projected sections out of the micro uneven parts existing in the surfaces are contacted, so that the true contact area is smaller than the apparent one. Consequently, high pressure will be applied on this true contact area, i.e., the projected sections that are contacted, to generate the deposition of contacted metals. As a result, cohesion abrasion is produced by which the soft metal is torn off and moved to the hard metal. Further, in the case where materials of different hardness are contacted, or in the case where hard particles are contained in one side even when soft metals are contacted, the soft metal is mechanically sheared by the hard metal to produce scratch abrasion.
Such abrasion phenomena depend heavily on the hardness of each metal material to be contacted, the bonding abilities of those metals and others, and abrasion phenomena of sliding contact materials also become remarkable basically in proportion to the contact pressure, so the abrasion phenomena can be reduced by the hardening of materials. However, abrasion phenomena also greatly change according to the change of temperature and humidity and the existence of any corrosive component, organic vapor, dust and the like when the materials are contacted. And since this change of abrasion phenomena is the change of the contacting states at the contact part, it will cause increasing in contact resistance to affect greatly the stable maintenance of contact resistance.
Abrasion phenomena mentioned above are concretely induced between a commutator and a brush when a cladding composite material using a sliding contact material is built into a small direct-current motor as a commutator and the motor is driven at high speed rotation. That is, the sliding contact material constituting the commutator is subjected to contact friction for a long stretch of time and frictional heat is also added, resulting in inducing cohesion abrasion and scratch abrasion as mentioned above in the combined state. Consequently, the surface of the sliding contact material is shaved by the abrasion phenomena to produce abrasion powder, which powder causes to increase contact resistance, make conduction short by filling up gaps between commutators with the abrasion powder or be attributable to generate noise.
Moreover, if the abrasion phenomena proceed further, in a cladding composite material using a sliding contact material, the metal, i.e., the sliding contact material that is provided on the surface layer of the cladding composite material is broken by abrasion and the abrasion will reach to the base material under the composite material. When such an abrasion state is made, because the base material, which is easily oxidized, is exposed, all sorts of electrical troubles may be caused by the metal oxide of the base material. Accordingly, when a so-called two-layer or three-layer cladding composite material is constituted and used as a commutator, it is an extremely important subject to improve an alloy material composing each layer.
Now, in recent years, as a sliding contact material that is used in a commutator for a small direct-current motor which is used for loading of taking a CD in and out in a CD player or used for sending a pick to move a lens for reading signals of a CD, and a sliding contact material that is further used in a commutator for a small direct-current motor which is used in household electrical appliances that are driven with a rechargeable battery, a two layer cladding composite material in which a Agxe2x80x94Cd alloy containing 1 to 2 wt. % Cd and the balance of Ag is used in the surface layer, and Cu or a Cu alloy is used in the base layer (e.g., Ag 99-Cd 1/Cu), a two-layer cladding composite material in which a Agxe2x80x94Cdxe2x80x94Ni alloy containing 1 to 2 wt. % Cd, 0.01 to 0.70 wt. % Ni and the balance of Ag is used in the surface layer, and Cu or a Cu alloy is used in the base layer (e.g., Ag 97.7-Cd 2-Ni 0.3/Cu) and others are used. The xe2x80x9calloy composition/Cuxe2x80x9d described in the parentheses mentioned above means a cladding composite material constituting two layers and the xe2x80x9c/xe2x80x9d means the interface between the surface layer and the base layer. Further, the numerals described after the elements of alloy compositions mean the values in weight percent.
Such Agxe2x80x94Cd alloy and Agxe2x80x94Cdxe2x80x94Ni alloy are materials that are very excellent in electrical functions, hardness and low contact resistance properties, and are disclosed in, for example, Japanese Patent Publication No. Hei 2-60745 as a sliding contact material comprising a Ag alloy that contains at least one of Sn and Cd in 1 to 5 wt. % and the balance of Ag for a commutator in a small direct-current motor. However, considering environmental problems in these days, it is not desirable to produce and use sliding contact materials containing Cd that is considered to be a harmful substance.
As other typical alloys, Agxe2x80x94Cu alloys, Agxe2x80x94Cuxe2x80x94Cd alloys are also used. However, these sliding contact materials have low contact resistance at the initial stage of their usage, but the contact resistance is subject to secular change. Consequently, they have such a problem that the worth of a product using a rechargeable battery, including a shaver, is inferior. That is, in the case where a sliding contact material of these alloys is used in a motor, because the contact resistance becomes high with the passage of time, the starting voltage of the motor becomes high to lower the electromotive force of the battery, which cause a problem that the motor is not started up. As a result, the frequency of charging the battery is also increased and the battery itself shows a tendency of shortening its life.
Further, for example, in Japanese Patent Application Laid-open No. Sho 58-104140, a sliding contact material of Agxe2x80x94Zn-based alloys is disclosed in which 1 to 10 wt. % Zn and 0.5 to 1.0 wt. % at least one metal selected from the group of Te, Co, Ni, Cu, Ge, Ti and Pb are added in Ag. This sliding contact material contains Te, Co, Ni, Cu, Ge, Ti and Pb in order to retard the oxidation of Zn, maintain the sulfuration resistance and lubricity of the sliding contact material, improve the abrasion resistance and stabilize the low contact resistance because of the characteristics that these metals are oxidized more easily than Zn. However, this sliding contact material also has low contact resistance at the initial stage similarly to Agxe2x80x94Cu alloy and the like, but the contact resistance changes with the passage of time and becomes high as the period of its usage is prolonged.
Moreover, in Japanese published unexamined application 8-260078, sliding contact materials of Agxe2x80x94Zn alloys and Agxe2x80x94Znxe2x80x94Ni alloys are disclosed. These materials also have low contact resistance, but do not gain a sliding contact material in which abrasion phenomena are controlled to such a degree as to improve the life of a motor.
As mentioned above, it is difficult to say that conventional sliding contact materials can correspond sufficiently to the specifications of loading and those of sending a pick in recent downsized CD players. With a CD player is downsized, a motor used in the inside of the player is also downsized, but the specifications themselves of loading in the CD player need the same torque as that needed conventionally without reference to the size of a motor. From that reason, even if a motor is downsized, a downsized motor having necessary torque is realized by operating the motor at high velocity revolution of 10,000 rpm or more and through a gear. However, because the properties of conventional sliding contact material are insufficient in the area of this high velocity revolution of 10,000 rpm or more, a more durable sliding contact material is strongly needed.
Accordingly, the present invention is aimed at providing a sliding contact material that has an alloy composition containing no harmful substance like Cd, especially excellent contact resistance properties, electrical functions that are good and is not subject to secular change, and abrasion resistance practically bearing comparison with conventional sliding contact materials, and further aimed at lengthening the life of a motor by the use of a sliding contact material having such excellent properties as a commutator for a small direct-current motor.
The present inventors have devoted themselves to the study and found that the above-mentioned subjects would be solved by the usage of a sliding contact material, of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy produced in such a method that 0. 7 to 3.0 wt. % Ni powder, an additive of Li2CO3 powder corresponding to 0.01 to 0.50 wt. % Li after being converted to metal and the balance of Ag powder were mixed and stirred to form a uniformly dispersed mixture, then the mixture was formed and sintered.
The sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy of the present invention is an alloy of Agxe2x80x94Ni series in which Ni particles are dispersed in a Ag matrix and Li2CO3 is moderately dispersed in the alloy. Li2CO3 dispersed in this material forms LiOH.H2O on the surface of the material during its sliding and the formed LiOH.H2O becomes a coating and serves as a lubricant on the sliding part to lower the frictional resistance of the material. As a result, the abrasion resistance of the material is improved.
Conventional sliding contact materials, for example, Agxe2x80x94Zn alloys, Agxe2x80x94Cu alloys and the like are also aimed at controlling abrasion phenomena by forming oxide bands of ZnO and CuO, but these alloys produce ZnO and CuO in surplus at the contact part with the passage of time when they are let alone in the air, resulting in increasing the contact resistance of the material conversely. In particular, when CuO having low electric conductivity is produced in surplus, the contact resistance is remarkably increased. Even in case of ZnO that is electrically conductive, its excessive production will increase the contact resistance.
On the other hand, in the sliding contact material of the present invention, Ni metal particles in Ag matrix slightly form NiO on the polar surface, but NiO does not cover all the surface of the contact because Ni exists as metal particles in the material. In addition, since Li2Co3 dispersing in the material is small in quantity as it is converted to 0.01 to 0.50 wt. % Li metal, it does not have as much influences as to increase the contact resistance.
Moreover, since the sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy of the present invention is also produced by the so-called powder metallurgy method, Ni metal particles and Li2CO3, which exist in Ag matrix, are dispersed with extreme uniformity. In the dissolution method, however, Agxe2x80x94Ni series alloys having the same compositions as those in the present invention cannot be formed. From that reason, in the present invention, the improvement of the stability of the contact resistance and the abrasion resistance, which could not be made by sliding contact materials of conventional Agxe2x80x94Znxe2x80x94Pdxe2x80x94Cuxe2x80x94Ni alloys and the like, can be achieved at the same time without containing Cd.
Ni metal particles in this sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy of the present invention mainly performs a role of improving the abrasion resistance of the sliding contact material. If the amount of Ni is less than 0.7 wt. % when mixed as Ni powder, the effect of improving the abrasion resistance with Ni metal particles tends to decrease, and if the amount is over 3.0 wt. %, the abrasion resistance will be excessively improved to wear the brush, resulting in the shortening of the durable life of the motor. Mixing Ni powder in 0.7 to 2.0 wt. % will be able to make the properties of the sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy related to the present invention most excellent.
In the sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy of the present invention, the amount of Ni contained in the material is 0.7 to 3.0 wt. %. In the case where such sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy is daringly formed by fusion casting, because Ag and Ni are hardly dissolved each other when they are fused, they are separated in two phases and exist separately in the fused state respectively so that Ni is upper side and Ag is lower side in a crucible. Accordingly, even if they are cast, only such a Agxe2x80x94Ni alloy as Ni is segregated can be obtained. That is, the sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy of the present invention cannot be formed by the dissolution method. Thus, since the sliding contact material of an alloy of Agxe2x80x94Ni series of the present invention is formed by the powder metallurgy method, Ni particles in the material become to be in the state of dispersed with extreme uniformity in Ag matrix and function sufficiently to improve the abrasion resistance.
Further, Li2CO3 that is dispersed in the sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy of the present invention becomes to be LiOH.H2O at the sliding part, that is, the contact surface and works as a lubricant. If the amount of the dispersed Li2CO3 is less than 0.01 wt. % Li after being converted to metal, it tends to decrease in exerting the function as a lubricant, and if the amount is over 0.5 wt. %, the stability of the contact resistance of the sliding contact material tends to lower as well as the lowering of its processability. About Li2CO3, mixing Li2CO3 powder in the rate of 0.05 to 0.20 wt. % Li after being converted to metal will be able to make the properties of the sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy related to the present invention most excellent.
The present inventors have performed all sorts of studies on additives for the sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy and found that the subjects of the present invention could be achieved by adding La2O3 in addition to Li2CO3. To be concrete, it is a sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy produced in such a method that 0.7 to 3.0 wt. % Ni powder, additives of Li2CO3 powder corresponding to 0.01 to 0.50 wt. % Li after being converted to metal and La2O3 powder corresponding to 0.01 to 1.00 wt. % La after being converted to metal, and the balance of Ag powder were mixed and stirred to form a uniformly dispersed mixture, then the mixture was formed and sintered.
This La2O3 disperses in the material similarly to Li2CO3, and La2O3 particles themselves work as lubricant and further exist in not only Ag matrix but also the inside of Ni metal particles to contribute to improve the abrasion resistance of the material together with the synergistic effect of improving the abrasion resistance of Ni metal particles. If the amount of the dispersed La2O3 is less than 0.01 wt. % La after being converted to metal, it cannot obtain the synergistic effect with Ni metal particles, and if the amount is over 1.00 wt. %, the stability of the contact resistance of the sliding contact material tends to lower as well as the lowering of its proccessability. About La2O3, mixing La2O3 powder in the rate of 0.20 to 0.40 wt. % La after being converted to metal will be able to make the properties of the sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy related to the present invention most excellent.
Other oxides of rare earth elements, for example, Ce2O3, Sm2O3 and the like can be substituted for La2O3 in the sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy of the present invention. The reason why La2O3 is adopted in the sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy of the present invention is that La2O3 is resourceful and easily available.
In the case where the sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy of the present invention is used as a commutator in a motor, in order to make more suitable material for a commutator, it is preferable to use Cu or a Cu alloy as a base material and to make a cladding composite material in which the sliding contact material of the present invention is buried under a part of the base material. In this manner, better soldering can be achieved in soldering treatment needed to electrically connect a commutator and processability of the material for forming it in a commutator shape is also improved. Moreover, because the thickness of the sliding contact material of the present invention, which material is buried under the base material in accordance with a motor to be used, can be controlled by adopting a form of a cladding composite material, an expensive sliding contact material can be restricted to use partly, leading to an economically favorable result.
In case of a cladding composite material as mentioned above, part of the buried sliding contact material which is bared to the surface is easily corroded because of being exposed to the air. Accordingly, in a cladding composite material in which the sliding contact material of the present invention is buried under the part of the base material of Cu or a Cu alloy, it is preferable to coat at least part of the surface of the sliding contact material with Au or a Au alloy. Though Au or a Au alloy is known to be excellent in corrosion resistance and to be a good material for a sliding contact material to realize low contact resistance, it is economically disadvantageous to use them in large quantities because of its expensiveness. Accordingly, it is intended to retard the increase in the cost by coating a part with Au or a Au alloy and further to prevent the corrosion in the sliding contact material of Ni metal particle-dispersed-type Agxe2x80x94Ni-based alloy related to the present invention. If a cladding composite material like this is used for a commutator of a motor, good driving of the motor becomes possible because of the excellent contact resistance property of Au or a Au alloy at the initial stage of its usage, and even if Au or a Au alloy is broken down due to abrasion, because the sliding contact material of the present invention exists in the inner part, it is possible to further use the motor continuously.
Moreover, if a so-called two-layer or three-layer cladding composite material of the present invention as mentioned above is used in a small direct-current motor as a commutator, the low contact resistance can be stably realized, the change with the passage of time is small, and there is no trouble due to abrasion powder, consequently the small direct-current motor can be driven with low starting voltage. Hereby, when the cladding composite material is used for loading or sending a pick in a CD player, the life of the small direct-current motor itself can be prolonged.